An orbital defense platform[4] (ODP),[5] also known as an orbital weapon platform,[6] is a class of large UNSC space stations used to defend locations of great importance, typically planets of a high strategic value. They have the hull classification symbol of OWP.

Orbital defense platforms are frequently placed in geosynchronous orbit and deployed in groups, or clusters, of two to five platforms. The well-equipped and large bridge of an ODP is suitable for directing large-scale engagements. Such was the case when Fleet Admiral Sir Terrence Hood directed UNSC forces from an ODP during the Battle of Earth. These platforms are capable of destroying even the largest of Covenant capital ships but are vulnerable to Covenant boarding craft which can avoid the heavy slugs and latch onto the stations. Thus, it is up to the UNSC personnel within the stations to repel any boarders.[6]

Internally, the stations are remarkably spacious, complete with high ceilings and a monorail system to move personnel and equipment. In addition to numerous small craft bays, there are two docks for larger ships, such as freighters, or even frigates. These docking bays could possibly be used to hold Longsword fighters used to protect the station from hostile spacecraft. The design incorporates many features against potential hostile boarding action, including security stations and small arms racks situated at strategic locations.[7]

The primary and crucial armament is a Mk. V "Super" Magnetic Accelerator Cannon capable of causing near-fatal damage to even a shielded Covenant capital warship with a single slug. The slug can weigh up to 3,000 tons and is launched at 4% the speed of light,[8][9][note 1] ensuring the complete annihilation of the ODP's target.[7] As with starship-mounted MACs, a platform's cannon is commonly controlled by an AI. Due to the immense power of the MAC, a large recoil dampening system is situated at the base of the cannon.[6] Additionally, the platforms are armed with a number of point-defense guns for close-in defense against enemy munitions, fighters, and boarding craft.[3]

ODPs do not possess the power generation equipment necessary to fire their main armament. Instead, they receive power from ground-based generators. An effective strategy to neutralize the formidable ODPs would be to disable the generators via ground assault. This would disable the platform's weaponry without destroying it, allowing this valuable prize to be captured or destroyed outright. The Covenant used this strategy during their invasion of Reach, dropping troops to the planet's surface and destroying these generators. This paved the way for Covenant forces to completely overwhelm the UNSC forces in the space battle over Reach.

By August 2552, Reach was protected by twenty ODPs. At least one of these platforms was in place by May 2517;[10] a number were online by 2527.[11] During the Fall of Reach, the orbital defense platforms inflicted massive losses on the Covenant armada in the opening stages of the battle, as well as providing fire support for ground forces on Reach. Large numbers of UNSC forces were deployed to protect the orbital platforms' planetside power generators, but they ultimately fell against determined enemy assaults. The disabling of the defense platforms was a major factor on the battle's outcome.[12]

The outbreak of the Human-Covenant War prompted a massive expansion of Earth's defenses. Even as late as the Battle of Reach preparations had not yet been completed; a series of new platforms above Earth were set to come online on September 14.[13] By October 2552, some 300 ODPs were operational.[7] Many of Earth's ODPs were named after points of interest that they roughly kept station over.

During the Didact's attack on Earth in mid-2557, ODPs defended Earth from the Didact alongside the ships of the UNSC Home Fleet. However, the Didact's ship Mantle's Approach was too heavily shielded for even the ODPs Super MACs and as a result, the Master Chief had to destroy the ship from the inside with a HAVOK nuclear warhead.[15]

Several orbital defense platforms held station above the Outer Colony of Meridian, owing to the world's critical importance as an industrial supplier of the UNSC; these stations were lost to the Covenant during the Battle of Meridian in 2551.

All known orbital defense platforms over Earth are named after cities on Earth. It is also apparent that the stations' names are also based roughly on the region they orbit above: Cairo, Athens, and Malta, for example, are located rather close to each other geographically. However, the stations' locations do not directly correspond to the planetside locations of the cities, as the other stations in Cairo's battle cluster can easily be seen with the naked eye, whereas their namesakes are hundreds of kilometers apart.

The fact that there are 300 ODPs orbiting Earth could be a reference to the Battle of Thermopylae, in which 300 Spartans used the phalanx formation to hold their own against an army of more than 200,000 Persians.

Two weeks after the Fall of Reach, when the UNSC Security Council met at HIGHCOM in Sydney, it is mentioned by Colonel Ackerson that most of Earth's ODPs would take ten days before becoming fully operational.

The kinetic energy of a single shell from an orbital defense platform would be equivalent to almost51.6 gigatons of TNT, 1,031 times the yield of the real world's largest nuclear weapon currently tested.

The orbital defense platforms had entered the conceptual stage sometime around the 2490s, as Avery Johnson mentions that they were being proposed around the same time he shipped out for boot camp.